Novel Treatment of Excitotoxicity: Targeted Disruption of Intracellular Signalling From Glutamate Receptors

Glutamate

Excitotoxicity

Glutamate receptor and excitotoxicity Types of Glutamate receptor

mGluRs  G-protein-coupled membrane receptors  Downregulate K + channel and upregulate non-selection cation channel  Inhibit GABA receptor activity and potentiate iGluR function  Mediate neuronal plasticity, nociception, pain and neurodegeneretion

iGluRs Ligand-gated ion channels, permeable to Na +, K + or Ca 2+ Mediate synaptic plasticity related to much of the toxicity induced by glutamate

AMPA receptors Permeable to Na+, K+ Also permeable to Ca2+ unless it contain GluR2 Loss of GluR2 implicated in delayed death of neurones in ischemia Structure of GluR2

NMDA receptors Highly permeable to Ca 2+ and Na + Calcium transients responsible for the physiologic effects of NMDAR signalling Calcium transients also trigger excitotoxic death

Kainate receptors Share many of the same structural characteristics as NMDA and AMPA receptors Until recently, little was known about the functional and physiological roles of kainate receptors in the mammalian CNS

Calcium and neurotoxicity Localised increases in [Ca 2+ ] i trigger physiological events Excessive Ca 2+ loading activates processes that lead to cell death Neurotoxicity mediated by glutamate receptors is largely calcium dependent

Calcium and neurotoxicity Calcium load hypothesis Neurodegeneration is simply a function of the quantity of entering the cell Neurodegeneration is simply a function of the quantity of entering the cell However, some studies show that the calcium channel blockers can prevent Ca 2+ accumulation but not neurotoxicity during anoxia However, some studies show that the calcium channel blockers can prevent Ca 2+ accumulation but not neurotoxicity during anoxia

Calcium and neurotoxicity Source specific hypothesis Ca 2+ toxicity occurs not simply as a function of increased Ca 2+ concentration, but is instead linked to the route of Ca 2+ entry and the distinct second messenger pathways that are activated as a result. Ca 2+ toxicity occurs not simply as a function of increased Ca 2+ concentration, but is instead linked to the route of Ca 2+ entry and the distinct second messenger pathways that are activated as a result. Showing that Ca 2+ loads produced by voltage- sensitive Ca 2+ channels were not harmful whereas similar [Ca 2+ ] i increases via NMDARs were toxic Showing that Ca 2+ loads produced by voltage- sensitive Ca 2+ channels were not harmful whereas similar [Ca 2+ ] i increases via NMDARs were toxic

Postsynaptic organisation Postsynaptic density (PSD) PSD PSD is a multiprotein complex containing membrane proteins, signaling molecules and core PSD proteins

Postsynaptic organisation Membrane receptors and proteins mGluRs, iGluRs mGluRs, iGluRs Cell junction protein Cell junction protein

Postsynaptic organisation Enzymes and modulators Src-kinase, CaMKII, PKC, phosphatase calcineurin nNOS, SPAR, SynGAP

Postsynaptic organisation Cytoskeletal and scaffolding proteins actin, fodrin, tubulin and neurofilaments actin, fodrin, tubulin and neurofilaments Spectrin,  -actinin-2, AKAP 79 and PDZ-containing protein Spectrin,  -actinin-2, AKAP 79 and PDZ-containing protein

Neurotoxic signalling by glutamate receptors within the PSD Neurotoxic effects of AMPAR signalling GluR2 hypothesis GluR2 hypothesis

Neurotoxic signalling by glutamate receptors within the PSD Neurotoxic effects of NMDAR signalling Role of PSD-95 Role of PSD-95

Targeting intracellular signal pathways Strategies for treating excitotoxic damage oNMDAR and AMPAR blockers? oParticular receptor subunits antagonists oLow affinity blockers oTargeting the specific intracellular signal pathways and uncoupling glutamate receptors from their potentially neurotoxic downstream effectors

Future directions Limitations of the use of peptides and small proteins Protein tranduction domains oCross cell membranes independent of specific receptors or transporters oEnsure efficient delivery of attatched proteins into cells and across the BBB oParticularly suited to the narrow therapeutic window offered during stroke